Does it give you any benefit in summer with the top down?
Not with the top down, but when I had a Griff it was a definite benefit during the summer when the weather was too crappy to take the top off. Like most summers really, apart from the last one
I keep it on with the roof down and it's worth having. It's a late car though and get the impression from other comments that earlier systems weren't as effective.
On hot rainy days its very useful, also helps the windows demist quickly in winter.